Alzheimer's Disease (AD) is a progressive neurodegenerative disorder marked by loss of memory, cognition, and behavioral stability. AD afflicts 6-10% of the population over age 65 and up to 50% over age 85. It is the leading cause of dementia and the third leading cause of death after cardiovascular disease and cancer. There is currently no effective treatment for AD. The total net cost related to AD in the U.S. exceeds $100 billion annually.
AD does not have a simple etiology, however, it has been associated with certain risk factors including (1) age, (2) family history and (3) head trauma; other factors include environmental toxins and low levels of education. Specific neuropathological lesions in the limbic and cerebral cortices include intracellular neurofibrillary tangles consisting of hyperphosphorylated tau protein and the extracellular deposition of fibrillar aggregates of amyloid beta peptides (amyloid plaques). The major component of amyloid plaques are the amyloid beta (A-beta, Abeta or Aβ) peptides of various lengths. A variant thereof, which is the Aβ1-42-peptide (Abeta-42), is believed to be the major causative agent for amyloid formation. Another variant is the Aβ1-40-peptide (Abeta-40). Aβ is the proteolytic product of a precursor protein, beta amyloid precursor protein (beta-APP or APP).
Familial, early onset autosomal dominant forms of AD have been linked to missense mutations in the β-amyloid precursor protein (β-APP or APP) and in the presenilin proteins 1 and 2. In some patients, late onset forms of AD have been correlated with a specific allele of the apolipoprotein E (ApoE) gene, and, more recently, the finding of a mutation in alpha2-macroglobulin, which may be linked to at least 30% of the AD population. Despite this heterogeneity, all forms of AD exhibit similar pathological findings. Genetic analysis has provided the best clues for a logical therapeutic approach to AD. All mutations found to date, affect the quantitative or qualitative production of the amyloidogenic peptides known as Abeta-peptides (Aβ), specifically Aβ42, and have given strong support to the “amyloid cascade hypothesis” of AD (Tanzi and Bertram, 2005, Cell 120, 545). The likely link between Aβ peptide generation and AD pathology emphasizes the need for a better understanding of the mechanisms of Aβ production and strongly warrants a therapeutic approach at modulating Aβ levels.
The release of Aβ peptides is modulated by at least two proteolytic activities referred to as β- and γ-secretase cleavage at the N-terminus (Met-Asp bond) and the C-terminus (residues 37-42) of the Aβ peptide, respectively. In the secretory pathway, there is evidence that β-secretase cleaves first, leading to the secretion of s-APPβ (sβ) and the retention of a 11 kDa membrane-bound carboxy terminal fragment (CTF). The latter is believed to give rise to Aβ peptides following cleavage by γ-secretase. The amount of the longer isoform, Aβ42, is selectively increased in patients carrying certain mutations in the region of a particular gene coding in a particular protein (presenilin), and these mutations have been correlated with early-onset familial AD. Therefore, Aβ42 is believed by many researchers to be the main culprit of the pathogenesis of AD.
It has now become clear that the γ-secretase activity cannot be ascribed to a single protein, but is in fact associated with an assembly of different proteins.
The gamma (γ)-secretase activity resides within a multiprotein complex containing at least four components: the presenilin (PS) heterodimer, nicastrin, aph-1 and pen-2. The PS heterodimer consists of the amino- and carboxyterminal PS fragments generated by endoproteolysis of the precursor protein. The two aspartates of the catalytic site are at the interface of this heterodimer. It has recently been suggested that nicastrin serves as a gamma-secretase-substrate receptor. The functions of the other members of gamma-secretase are unknown, but they are all required for activity (Steiner, 2004. Curr. Alzheimer Research 1(3): 175-181).
Thus, although the molecular mechanism of the second cleavage-step has remained elusive until now, the γ-secretase-complex has become one of the prime targets in the search for compounds for the treatment of AD.
Various strategies have been proposed for targeting γ-secretase in AD, ranging from targeting the catalytic site directly, developing substrate-specific inhibitors and modulators of γ-secretase activity (Marjaux et al., 2004. Drug Discovery Today: Therapeutic Strategies, Volume 1, 1-6). Accordingly, a variety of compounds were described that have secretases as targets (Larner, 2004. Secretases as therapeutics targets in AD: patents 2000-2004. Expert Opin. Ther. Patents 14, 1403-1420).
Indeed, this finding was supported by biochemical studies in which an effect of certain Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) on γ-secretase was shown (US 2002/0128319; Eriksen (2003) J. Clin. Invest. 112, 440). Potential limitations for the use of NSAIDs to prevent or treat AD are their inhibition activity of cyclooxygenase (COX) enzymes, which can lead to unwanted side effects, and their low CNS penetration (Peretto et al., 2005, J. Med. Chem. 48, 5705-5720). More recently the NSAID R-flurbiprofen, an enantiomer lacking Cox-inhibitory activity and related gastric toxicity, has failed in large phase III trial since the drug did not improve thinking ability or the ability of patients to carry out daily activities significantly more than those patients on placebo.    WO-2009/103652 relates to 1H-1,2,4-triazol-3-amine derivatives as modulators for Aβ;    WO-2009/032277 relates to heterocyclic compounds useful as γ secretase modulators;    WO-2010/010188 relates to [1,2,4]triazolo-[1,5-a]pyridine compounds, useful for the treatment of degenerative joint diseases and inflammatory diseases;    WO-2010/098495 relates to imidazolylpyrazine derivatives as therapeutic agents for AD;    WO-2008/099210 relates to piperazine derivatives for treatment of AD and related conditions; and    WO-2008/100412 provides compounds useful for treating diseases associated with the deposition of β-amyloid peptide in the brain.
There is a strong need for novel compounds which modulate γ-secretase activity thereby opening new avenues for the treatment of AD. It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. The compounds of the present invention or part of the compounds of the present invention may have improved metabolic stability properties, improved central brain availability, improved solubilities, or reduced CYP inhibition compared with the compounds disclosed in the prior art. It is accordingly an object of the present invention to provide such novel compounds.